This invention relates to a security system.
Vehicle thefts are a matter of great concern to their owners, the police forces and to the insurance companies who have to meet the cost of such thefts.
There are currently available various devices which are designed to act as a deterrent to a potential thief.
In existing arrangements it is possible for vehicles to be easily "push-started", `jump-started` or `hot-wired` and thus to defeat a system which may have been installed to inhibit ignition from taking place and the engine from being started.
U.S. Pat. No. 3,714,628 discloses a security system which, when applied to a vehicle, or other instrumentality, is intended to prevent unauthorized use of the vehicle or other device on which it is installed, and to prevent tampering with any component attached to the vehicle or device. For this purpose, the conventional cylinder and tumbler locks are omitted and replaced with a multiple contact plug and socket. The plug is associated with a special key unit, coded and matched with a special lock unit associated with the socket, itself associated with a door and/or with an ignition unit. With the special key unit, the owner may lock and unlock the doors and/or boot, start and drive the vehicle, and turn off the ignition, in much the same way as he would use the conventional key. Further, if another key unit or plug, or "jump wiring" is used in an attempt to unlock the doors of the car or to start the engine, an alarm will be initiated to announce that the vehicle or other instrumentality has been tampered with. The system is so constructed as to turn off all the major components, including the ignition, petroleum supply, and electrical current to the various components. The system also includes setting an alarm against motion and unauthorized entry. A serious drawback of the system is that a thief can probe the terminals in its socket with a multimeter to determine which leads are performing which functions in the electrical system of the vehicle, and then connect across the relevant terminals.
GB 2136620 discloses a security system to prevent unauthorised use of a motor vehicle or other device, the system comprising a crystal oscillator circuit with a crystal which is removable for keeping by an authorised user, the crystal defining the frequency of oscillation of the oscillator circuit, a filter circuit tuned to the frequency of the oscillator circuit, and some means disabling the device against operation except upon receipt of a signal from the oscillator circuit via the tuned filter circuit when the crystal is inserted in the oscillator circuit.
The disabling means includes a relay having three changeover switches, of which one is employed for internal latching, the second is employed to maintain a bonnet lock solenoid energized in the absence of the crystal and the third is employed to cut the power to the ignition coil and instead applies it to the horn for alarm purposes, until the key is inserted, whereupon the power is transferred to the ignition coil. The system interrupts the power supply to only one device essential to the starting of the engine, namely the ignition coil at its positive side, so that the engine can be relatively easily jump-started by running a jump lead from the battery positive terminal to the ignition coil positive terminal. Although the horn would be sounded upon such jump-starting, a thief would simply sever the power supply to the horn. Therefore the only happening which would alert, for example, a policeman to the fact that the vehicle was being stolen would be the horn sounding and this could be for only a very short time.
U.S. Pat. No. 4,278,963 discloses a system similar to that of GB 2136620 except that the secure control means consists of an additional mechanical key and a combination thumbwheel switch arrangement and that, instead of interrupting the power supply to the ignition coil it interrupts the supply to the starter solenoid.
The interruption of the supply to only the starter solenoid has the serious disadvantage that the vehicle can be push-started.
U.S. Pat. No. 4,546,266 describes a security system in which an electrical circuit, the starting circuit for a motor vehicle for example, is selectively enabled by a control circuit which includes a magnetic field responsive switching device. Proper positioning of a magnet which provides a field of the appropriate polarity and strength relative to the switching device results in the energization of a relay which has contacts connected in series with the circuit being controlled. In the motor vehicle application, operation of the ignition switch within a pre-determined time subsequent to removal of the magnet will latch the relay in the energized state. The system is relatively insecure in that a thief either arrives provided with another suitable magnet or traces the wiring to the magnetically operated switching device and bypasses the device, and in that it interrupts the power supply to the starter relay, so that the vehicle can be push-started. Another security weakness is the utilization of a "service" plug, thus increasing the opportunities open to a potential thief.